A wireless mesh network is a collection of wireless nodes that can communicate with each other over a direct link between two nodes and/or over a route that includes a number of relay nodes (hops) between two nodes. A wireless mesh network is a form of a wireless ad hoc network. Ad Hoc On-Demand Distance Vector (AODV) Routing (RFC 3561) is a routing protocol for ad hoc networks including wireless mesh networks.
As described in RFC 3561, the types of messages defined by AODV include route request messages and route reply messages. A route request message is used by a source node to find a route to a destination node. The route reply message is sent from the destination node to the source node to complete discovery of the route and make the route available.
There are a number of issues not addressed by the AODV protocol as it is currently defined in RFC 3561. For one, routes are established and maintained in an asymmetric manner from the source node to the destination node. That is, the source node may communicate to the destination node over unidirectional links in a route that includes a set of one or more relay nodes, and the destination node may respond to the source node over unidirectional links in another route that includes a different set of relay nodes. Maintenance of symmetric routes, over bidirectional links, is not addressed by AODV.
Asymmetric routes can affect the performance or quality of the wireless mesh network. For example, according to RFC 3561, a node is to ignore a route request message that is received over a link that is unidirectional. This has two detrimental effects: first, it is necessary to periodically check the links to determine whether a link is unidirectional; and second, because route requests over unidirectional links are to be ignored, there is less flexibility when selecting routes between the source and destination nodes. Moreover, when it is necessary to maintain an asymmetric route, it is also necessary to check each link individually to determine whether it is still available. Asymmetric routes also increase the size of (number of entries in) routing tables because they require an entry for each unidirectional link.